1. Field of the Invention
This invention relates to improvements in the process of hot dip metallic coating of carbon steel strip and sheet material with molten coating metals such as zinc, zinc alloys, aluminum, aluminum alloys and terne. More particularly, this invention relates to the preparation of carbon steel strip and sheet surfaces for coating by a preliminary treatment involving heating in a furnace heated by direct combustion of fuel and air therein and in an atmosphere containing gaseous products of combustion, under conditions which achieve optimum combustion efficiency, and optimum production rate through an increase in furnace heat input. Carbon steels which may be treated by the process of the present invention include compositions falling within the definition of carbon steel as set forth in Steel Products Manual, Carbon Sheet Steel, page 7 (May 1970), published by American Iron and Steel Institute. Coated carbon steel strip or sheet produced in accordance with the process of the invention can be produced to commercial quality, drawing quality or non-earring (normalized) quality specifications.
2. Description of the Prior Art
In the hot dip metallic coating of carbon steel strip and sheet material without a flux, it is necessary to subject the sheet and strip surfaces to a preliminary treatment which provides a clean surface free of iron oxide scale which is readily wettable by the molten coating metal and to which the coating metal will adhere after solidification thereof. Two types of in-line-anneal preliminary treatments are commonly used in this country, one being the so-called Sendzimir process (a detailed description of which may be found in U.S. Pat. No. 2,110,893, issued Mar. 15, 1938 to T. Sendzimir) and the other being the so-called Selas process (a detailed description of which may be found in U.S. Pat. No. 3,320,085, issued May 16, 1967 to C. A. Turner, Jr.).
The Sendzimir process has several disadvantages, among which are a limitation on the strip preheat temperature in the open end oxidizing furnace to about 800.degree.F in order to avoid over-oxidation; a requirement for a high strip temperature cycle in a strongly reducing atmosphere, thereby making it impossible to practice sub-critical annealing cycles; abrasive contact between the atmosphere-furnace hearth rolls and the oxidized strip which causes hearth roll pick-up and in turn causes strip dents and gouges, thereby lowering the quality of the finished product; and the necessity to provide a high hydrogen content (at least 20%) reducing furnace atmosphere, thereby increasing cost and creating a potential safety hazard. These disadvantages are substantially avoided in the Selas-type method in which surface contaminants are removed by a high-gradient, direct-fired strip heating with a complete absence of strip oxidation under conventional conditions.
The direct-fired Selas furnace is connected in sealed relation to a subsequent furnace containing a controlled atmosphere of hydrogen and nitrogen. This is advantageous in that the furnace system can be operated above atmospheric pressure by controlling the discharge rate of the direct-fired furnace combustion products, thus eliminating the hazard of air contamination of the hydrogen and nitrogen atmosphere by small furnace leaks. In the conventional Selas-type method the following conditions must be observed:
The fuel-to-air ratio must be regulated to produce at least about 3% excess combustibles, by volume, in the furnace atmosphere.
According to the above-mentioned Turner patent a substantial difference between the furnace temperature and the maximum strip temperature must be maintained, i.e. the furnace temperature is maintained above about 1315.degree.C (2400.degree.F) and the maximum strip temperature is not allowed to exceed about 760.degree.C (1400.degree.F) or a critical strip temperature value. In actual commercial practice furnace temperatures of about 1205.degree.C(2200.degree.F) and higher are now commonly used.
Since the atmosphere of gaseous products of combustion in the direct-fired Selas furnace is reducing to carbon steel under dynamic strip heating conditions, hydrogen contents of 5% or less by volume are adequate in the subsequent furnace having the controlled atmosphere of hydrogen and nitrogen.
The Selas-type direct-fired furnace may either be connected to a subsequent cooling section having a hydrogen and nitrogen atmosphere, or it may be connected to a subsequent furnace for further heating in a hydrogen and nitrogen atmosphere followed by cooling and/or holding. In either event, this is followed by a coating section, and the strip is brought approximately to the bath temperature and conducted beneath the level of the molten coating metal bath while still surrounded by the protective hydrogen-nitrogen atmosphere. The coating and finishing are carried out by any conventional method.
The process of the present invention is applicable to the second above-described type of Selas method, i.e. wherein a subsequent reducing furnace is provided, preferably of vertical configuration.
It has previously been considered essential that the strip leaving the direct-fired furnace be bright and non-oxidized in order to obtain satisfactory coating quality, in the conventional Selas-type process. This is effected by maintaining at least about 3% excess combustibles in the furnace atmosphere, and by controlling the maximum strip temperature relative to the thickness of the strip and the furnace temperature, so as to insure that no trace of oxidation occurs on the surface of the strip material.
While the Selas-type method has the above-mentioned advantages over the older Sendzimir method, nevertheless the Selas-type method does not realize optimum combustion efficiency and optimum production rate.